Water-soluble polymers are used for various applications and, in particular, as flocculants for processing industrial and urban waste water, dehydration of the sludges generated, as a thickener and as a treating agent for floors. Aqueous systems of such water-soluble polymers with a high solids content are in the form of a gel and have very high viscosities, which make them difficult to handle and store. The problem with which a person skilled in the art is faced is to produce such aqueous systems but which have both a high solids content and low viscosity.
Conventional processes for synthesizing these polymers comprise solution, reverse-suspension and reverse-emulsion polymerization. Solution and reverse-suspension polymerization lead to products in powder form which have the drawback of generating dust at the time of use, of dissolving poorly in water and of not being able to form aqueous solutions of polymers in high concentration which can be handled easily. Besides this inconvenient implementation which is intrinsic to the pulverulent form of the product, these two processes are handicapped in terms of production efficiency, on the one hand on account of the low monomer concentration used during polymerization, and on the other hand on account of a drying and/or grinding step leading to an increase in the cycle time and excessive energy consumption costs. As regards the reverse-emulsion polymerization process, this leads to a product containing a pollutant organic solvent and having unsatisfactory stability.
To overcome these drawbacks, a novel polymerization technique has been developed leading to aqueous dispersions of water-soluble polymers, the originality of which process lies in the presentation plan, i.e. they are free of pollutant solvent, do not generate any dust, are rapidly water-soluble, are of low viscosity, have a high polymer content, and are ready to use. However, this technique requires suitable polymer dispersants.
Certain authors have prepared cationic, amphoteric or nonionic water-soluble polymers by polymerization of water-soluble monomers in the presence of a polymer dispersant of low mass. EP 170 394 describes a dispersion of polymer gel particles which are greater than 20 .mu.m in size in a solution of poly(sodium acrylate) or poly(diallyldimethylammonium chloride) (poly(DADMAC)) as dispersant. However, this product has the drawback of having an increase in viscosity after a long period of storage. This viscosity can only be reduced after shear or stirring.
U.S. Pat. No. 183,466, EP 525,751 A1, U.S. Pat. No. 4,929,655, U.S. Pat. No. 5,006,590 and EP 657,478 A2 propose the case of precipitation-polymerization in saline medium of water-soluble monomers, the polymer of which precipitates in particle form and is then dispersed by means of stirring and stabilized by polymer dispersants of low mass, which are themselves soluble in saline medium.
Documents DE 4,216,167A1 and U.S. Pat. No. 5,403,883 describe a technique for obtaining salt-free dispersions, of low viscosity by polymerization in the presence of the dispersant poly(DADMAC), of a mixture of hydrophilic, hydrophobic and optionally amphiphilic monomers. On the same basis, EP 0,670,333 describes dispersions of polymers crosslinked by adding a crosslinking agent such as N-methylolacrylamide or N,N-methylenebisacrylamide to the mixture of monomers to be polymerized. Post-addition of the same dispersant allows a decrease in the viscosity of the systems described above (CA 3,123,460). However, the effect of his is to increase the level of dispersant and to lead to a dispersion having a low level of dispersed polymer.
The prior art shows that the viability of a process for the synthesis of a dispersion of water-soluble polymers is based on the production of a salt-free product of low viscosity having a high level of dispersed polymer which must be of high mass, and a low level of dispersant which must be of low mass, while at the same time giving good performance levels in the final application.